The present invention relates generally to security tags for use with an electronic security system for detecting the unauthorized removal of articles from an area and, more particularly, to such security tags which include electrostatic protection.
The use of electronic article surveillance (EAS) systems for detecting and preventing theft or other unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, EAS systems utilize a label or security tag containing an electronic circuit, such as an inductor/capacitor resonant circuit, which is secured to an article or the packaging for the article. A transmitter tuned to the frequency of the resonant circuit of the security tag is employed for transmitting electromagnetic energy into a protected or security area typically proximate to the exit of a retail establishment or other facility. A receiver, also tuned to the resonant frequency of the tag, is also located proximate to the protected area. The transmitter produces a continuous swept radio frequency field which is continuously received by the receiver. If an article containing a security tag enters the protected area, the resonant circuit within the tag resonates, providing a disturbance in the electromagnetic field which is detected by the receiver for activation of an alarm to alert security personnel.
In order to prevent accidental activation of an alarm by a person who has actually purchased an article having a security tag or who is otherwise authorized to remove an article having a security tag from a facility, security tags may be deactivated. One method of deactivating a security tag involves momentarily placing the tag near a deactivation device which subjects the tag to electromagnetic energy at a power level sufficient to cause the resonant circuit to short circuit. In order to avoid having the deactivation electromagnetic energy at a high power level, deactivatable security tags typically have one or more capacitor elements in which the dielectric between the plates of one or more of the capacitor elements is weakened or reduced so that the capacitor plates may be short circuited when exposed to relatively low power levels at the resonant frequency. The structure and operation of such deactivatable security tags is described in detail in U.S. Pat. Nos. 4,498,076 entitled "Resonant Tag and Deactivator for Use in Electronic Security System", and 4,728,938 entitled "Security Tag Deactivation System", each of which is incorporated herein by reference.
Other, more recently developed security tags are both activatable and deactivatable. Activatable/deactivatable security tags typically include a resonant circuit having at least two capacitors, each of which includes a weakened or reduced dielectric between the capacitor plates to facilitate short circuiting of the capacitors. The resonant circuits of activatable/deactivatable tags typically have an initial resonant frequency which is generally above the frequency range of the EAS system. When these tags are exposed to a sufficient level of electromagnetic energy at the initial resonant frequency, one of the capacitors becomes short circuited thereby shifting the resonant frequency of the security tag to a frequency within the frequency range of the EAS system. The security tag may be deactivated by exposing the resonant circuit to a sufficient level of electromagnetic energy at the new resonant frequency to short circuit the second capacitor thereby either preventing the resonant circuit from resonating at all or shifting the frequency of the resonant circuit beyond the frequency range of the EAS system. The structure and operation of activatable/deactivatable tags of this type is described in pending U.S. Pat. No. 5,081,445, entitled "Method for Tagging Articles Used in Conjunction with an Electronic Article Surveillance System, and Tags or Labels Useful in Connection Therewith", and in pending U.S. Pat. No. 5,103,210, entitled "Activatable/Deactivatable Security Tag for Use with an Electronic Security System", both of which are incorporated herein by reference.
While deactivatable and activatable/deactivatable security tags have been shown to be very effective when utilized in EAS systems, they have been found to suffer from certain drawbacks. Security tags of this type are typically formed of a flexible, substantially planar dielectric substrate having a first conductive pattern on a first side and a second conductive pattern on the second side, the conductive patterns together establishing the resonant circuit with the substrate forming the dielectric between the plates of the capacitor(s). Under certain circumstances, an electrostatic build-up occurs between the two sides of the substrate resulting in the charging of the capacitor(s). In some cases, the electrostatic build-up results in a discharge of sufficient energy to cause a premature breakdown of the dielectric between the plates of one or more of the capacitors thereby short circuiting one or more of the capacitors and either prematurely activating the security tag (in the case of the activatable/deactivatable tag) or prematurely deactivating the security tag. In either event, such security tags are not usable in an EAS system.
The present invention comprises a security tag which includes static dissipation means, such as frangible connection means or conductive frame member positioned on the substrate for draining any static electricity charge build-up from the substrate. In a preferred embodiment, the frangible connection means or frame member is employed for electrically connecting together both plates of at least some and preferably all of the capacitors of the security tag at least during manufacture, shipment and storage of the tag. The static dissipation means or frame member effectively prevents electrostatic build-up and discharge between the two sides of the dielectric substrate and, therefore, prevents the premature short circuiting of the capacitors. In the preferred embodiment, when the security tag is to be used, the connection between at least one plate of each of the capacitors and the frame member is broken to permit normal use of the security tag in conjunction with an EAS system.